The invention is based on a vehicle brake system as generally defined hereinafter.
In order to attain the shortest possible vehicle stopping distances of a vehicle in its track without undue swerving, it is known to incorporate brake pressure regulating valves in the brake lines leading to the rear wheel brakes (U. S. Pat. No. 2,991,797 and German Offenlegungsschrift 32 22 798). When wheel stresses have been dynamically altered because of braking deceleration, these brake pressure regulating valves avoid excessively severe actuation of the rear wheel brakes. The brake pressure regulating valves are preferably installed in vehicles having a front wheel drive. Depending on the type of brake pressure regulating valve used, the theoretically maximum possible rear wheel braking forces are only partially attained.
Anti-skid brake systems are shown in U.S. Pat. No. 4,478,461 and 4,484,784 which include control for controlling the front and back brakes. Prior art systems have the advantage that with increasingly forceful pedal actuation, they limit braking pressures whenever the maximum possible friction between the wheel and the road surface at a given time has been attained. If a severely braked vehicle moves from a gripping surface to one that has less grip, such as ice, then in such anti-skid brake systems the braking pressures are reduced until such time as the maximum possible friction between wheel and road surface has again been attained. The known brake pressure regulating valves for rear wheel brakes can also be built into anti-skid brake systems, for the sake of their known advantages. However, building in a brake pressure regulating valve of simple embodiment may have the disadvantage, if the rear wheel brake pressure limitation has attained a maximum value and at least one front wheel of the vehicle is tending to skid, that the maximum possible friction between the rear wheels and the road surface cannot be exploited for decelerating the vehicle. In a particular type of anti-skid system, the driver is warned of tendencies toward skidding by means of vibrations at the pedal of the vehicle brake system. With anxious drivers, the result can be that even in an emergency, where the maximum possible braking deceleration is essential, they will not actuate the pedal as severely as the situation warrants. Building in known brake pressure regulating valves has the further disadvantage that it makes the vehicle brake system more expensive, 10 and it increases the number of locations at which leakage losses can occur.